This invention relates generally to chemical vapor deposition (CVD) and plasma-enhanced chemical vapor deposition (PECVD), and more specifically to an apparatus and method for preventing the premature mixture of reactant gas constituents in CVD and PECVD reactions before such mixture is desired in the reaction chamber.
In the formation of integrated circuits (IC""s), it is often necessary to deposit thin films or layers, such as films containing metal and metalloid elements, upon the surface of a substrate, such as a semiconductor wafer. One purpose of such thin films is to provide conductive and ohmic contacts in the circuits and to yield conductive or barrier layers between the various devices of an IC. For example, a desired film might be applied to the exposed surface of a contact or via hole on an insulating layer of a substrate, with the film passing through the insulating layer to provide plugs of conductive material for the purpose of making inter-connections across the insulating layer.
One well known process for depositing such films is chemical vapor deposition (CVD) in which a film is deposited using chemical reactions between various constituent or reactant gases. In CVD, reactant gases are pumped into the processing space of a reaction chamber containing a substrate. The gases react in the processing space proximate the substrate, resulting in one or more reaction by-products. The reaction by-products then deposit onto the substrate to form a film on the exposed substrate surface.
Another variation of the CVD process which is widely utilized is a plasma-enhanced CVD process or PECVD process in which one or more of the reactant gases is ionized into a gas plasma to provide energy to the reaction process. PECVD is desirable for lowering the temperatures that are usually necessary for a proper reaction with standard CVD. In PECVD, electrical energy is delivered to the gas or gases to form and sustain the plasma. For one such PECVD process, the susceptor containing the substrate and a planar element in the processing space, such as a gas supply element, are electrically biased to operate as RF electrodes for energizing one or more of the reactant gases into an ionized plasma. Such a method is commonly referred to as a parallel plate method because the susceptor and the other biased planar element are maintained generally parallel to one another to simulate biased electrical plates with the substrate positioned therebetween and parallel to the biased elements.
The reactant gases for CVD and PECVD processes are delivered to the processing space and substrate through a gas delivery system which provides the proper flow and distribution of the gases for the CVD process. Generally, such gas delivery systems contain gas-dispersing elements in the reaction chamber, such as gas injector rings or flat showerheads, which spread the entering reactant gases around the processing space to insure a uniform distribution and flow of the gases proximate the substrate. Uniform gas distribution and flow is desirable for a uniform and efficient deposition process, a dense plasma, and a uniformly deposited film. Since the gases utilized in CVD and PECVD processes are reactive, it is often necessary to use a separate dispersing element for each constituent gas in order to keep the gases segregated or unmixed prior to the processing space. Otherwise, if the gases mix prior to the processing space, premature deposition occurs inside the dispersing element and inside other sections of the gas delivery system, which hinders a uniform flow of the gas, degrades the deposition process and may contaminate the deposited film.
To maintain separate constituent gases, multiple, concentric gas injector rings have been utilized to prevent premature mixture and deposition prior to the processing space. However, multiple gas injector rings in the processing space make it difficult to utilize PECVD techniques because the rings interfere with the placement and action of the RF electrodes necessary for such PECVD techniques. Therefore, the rings detrimentally affect plasma generation.
Conventional RF PECVD processes generally utilize a biased, planar gas showerhead opposite a parallel, biased susceptor. One such PECVD process and apparatus is disclosed in U.S. Pat. No. 5,547,243, which is commonly owned with the present application. While such a technique produces suitable PECVD films, directing and dispersing all of the reactant gas constituents through available showerheads will produce premature mixing of the gases before the processing space and yield undesirable deposition inside of the showerhead, or in-line in the system before the showerhead. Therefore, for parallel plate PECVD, it has been necessary to disperse some gases through inlet ports other than the showerhead, yielding non-uniform flow of some of the gas constituents at the substrate, or interfering with plasma generation.
Accordingly, it is an objective of the present invention to reduce and generally prevent the premature mixture of reactant gases in CVD and PECVD reactions.
It is still another objective of the invention to prevent the deposition of film material in the gas delivery system and to provide a uniform flow and distribution of reactant gases to the processing space for the deposition process.
It is another objective of the invention to maintain the separation of the reactant gases and generally prevent their interaction until they are injected and mixed proximate the substrate.
It is a further objective generally to prevent such premature interaction and deposition in a PECVD process utilizing parallel plate electrodes without interfering with the RF plasma generation.
Accordingly, the present invention addresses these objectives and the shortcomings of the various CVD and PECVD apparatuses and processes currently available in the prior art.
The present invention prevents premature mixture of reactant gases in CVD and PECVD reactions and maintains a separation of reactant gases to prevent their interaction until they are injected and mixed in the processing space proximate a substrate. The present invention further provides a uniform flow and distribution of the reactant gases and is suitable for use with RF plasmas and PECVD processes without interfering with the plasma. Particularly, the present invention provides the necessary gas separation while being suitable for parallel plate PECVD processes.
The present invention comprises a generally circular, planar gas-dispersing manifold, preferably in the form of a planar showerhead, which is coupled to at least two different reactant gas lines for dispersing reactant gases into a chamber proximate a substrate. The showerhead has a first space therein which is operable for receiving and dispersing a first reactant gas, and further comprises a second space, which is isolated from the first space, and is also operable for receiving and dispersing a second gas independently of the dispersion of the first gas. The showerhead of the invention maintains a segregation between the reactant gases in the first and second spaces, and prevents a premature mixture of the gases before the gases enter the processing space. In that way, premature deposition in the gas delivery system and prior to the processing space is generally prevented.
To disperse the reactant gases passing through the inventive showerhead, the showerhead includes two separate pluralities of gas-dispersing passages, which are in communication with each of the respective gas spaces within the showerhead, but are isolated from each other. The dispersing passages have outlets which open at a face surface of the showerhead opposite the substrate. When the separate reactant gases are directed through the showerhead, no mixture occurs within the showerhead, and each of the reactant gases is dispersed independently to thus mix proximate the substrate, as desired. The gas-dispersing passages for each of the respective first and second spaces are positioned in cooperating grids around the lower face surface of the showerhead to uniformly disperse and mix the gases proximate the substrate.
In accordance with another aspect of the present invention, the showerhead has a generally planar, and thus compact, design which functions electrically as a parallel plate when biased with RF energy. Therefore, the inventive showerhead may be utilized for parallel plate PECVD processes without interfering with the plasma. As such, the reactant gases are dispersed separately and uniformly for a stable, uniform plasma and a uniform deposition of the film.
The showerhead of the invention has a planar first space positioned in a plane generally parallel with a planar second space and below the second space. That is, the second space is stacked above the first space in the showerhead. Gas is introduced into each of the respective spaces through ports that communicate with the spaces, and the reactant gases spread through the planar spaces to be uniformly dispersed proximate the substrate by the grids of dispersing passages.
In one embodiment of the invention, the first gas space comprises a plurality of elongated cylindrical passages which extend through the showerhead. The passages originate at one area on the periphery of the circular showerhead and extend to another peripheral area on an opposite side of the showerhead. The elongated passages are generally isolated from each other along their lengths, but are co-planar and extend next to each other to define the planar first space. The opposite ends of the elongated first space passages are each coupled to a peripheral coupler which has a single inlet port and a wide outlet port for simultaneously interfacing with each of the ends of the elongated passages. The two couplers provide gas simultaneously to each of the ends of the elongated passages so that gas introduced at the periphery of the circular showerhead is distributed uniformly in the first space and around the showerhead. The elongated passages generally angle out from each coupler to reach a maximum area of the showerhead face surface and then angle back to the opposite coupler.
The second space is an open cylindrical space above the first space elongated passages. A second reactant gas is introduced into the second space through two inlet ports positioned at opposite peripheral points on the showerhead. The ports for introducing the second gas are positioned at approximately a 90xc2x0 offset on the showerhead periphery from the peripheral first gas couplers so as not to interfere with the couplers for the first space.
One set of gas dispersing passages is arranged in a grid on the showerhead and communicates between the second gas space and the face of the showerhead so that the second gas may be delivered to the processing space. Each passage from the set extends from the second space, past the elongated first space passages, and opens at the showerhead face without intersecting the first space passages. In that way, the gases are kept segregated in the showerhead. Another set of dispersing passages, also in a grid arrangement, communicates with the elongated passages of the first space to deliver the gas therefrom.
In another embodiment of the present invention, the reactant gases are introduced into the center of the showerhead rather than at the periphery thereof. To that end, the showerhead includes a center stem having two passages and two inlet ports for the respective first and second gases. The center stem extends generally perpendicular to the plane of the showerhead and one of the gas inlet ports opens directly into the second space. Preferably a 90xc2x0 coupler is used to direct the incoming second gas parallel to the plane of the second space. The center stem may be biased with RF energy when desired for PECVD processing.
The first gas port communicates with a diametrical passage, located above the first and second spaces in the showerhead, which directs the gas out to the periphery of the showerhead. The first gas space comprises a peripheral channel which distributes the first gas around the periphery of the showerhead. Gas distribution fingers, each opened at one end thereof, are coupled to the channel and extend toward a diameter line of the showerhead to terminate proximate the diameter line. The fingers are co-planar and are generally parallel to one another, with one set of fingers distributing gas to one half of the showerhead and another set of fingers distributing gas to the other half of the circular showerhead.
Sets of gas dispersing passages are arranged in interacting grids, similar to the embodiment previously described, and the dispersing passages communicate between the respective first and second gas spaces and the showerhead face. The second space passages extend between the fingers of the first space so as not to mix the reactant gases prior to their dispersion at the face of the showerhead and proximate the substrate.
The invention thus provides a segregated, and uniform distribution of the reactant gases while reducing deposition of film material prior to entry of the reactant gases into the processing space containing the substrate. In that way, an efficient gas flow is achieved, premature deposition is prevented, and the likelihood of contamination from deposition within the showerhead is reduced. Furthermore, gas segregation may be maintained during RF PECVD process.